The invention relates to flowable material dispensers and more particularly to depositing solder in a desired pattern on the surface of a workpiece.
Solder pads are often deposited in a desired pattern on a circuit board using a stencil printing system. Screens can be used in place of stencils, and solder printing systems have also been used to apply a pattern of adhesive pads to circuit boards. Typically a line of solder paste is dispensed onto the upper surface of the stencil, and a squeegee blade is used to spread the solder paste across the stencil and cause it to fill up the openings in the stencil and contact a circuit board positioned directly under the stencil. The circuit board is then lowered from the stencil, the board carrying solder pads in the desired pattern on its upper surface.
Cameras can be used to automatically inspect the deposited solder pads and provide information used to determine whether the pads are acceptable and whether process conditions should be changed. e.g., Chung, C-W, et al. "Closed Loop Process Control for Solder Paste Stencil Printing," pp. 485-491, describes viewing solder pads with a paste inspection system that determines such things as solder deposit area, solder deposit volume, average height of the solder paste deposit, border violation of the solder paste deposit, and the x and y position of the solder paste deposit. Chung describes the ability to use this information by a knowledge base/inference engine to control process parameters such as print speed, separation speed, print gap, squeegee pressure, paste dispense rate, and clean screen rate. Bryant, S., et al., "Neural Network Machine Vision for SMT Solder Paste Inspection," pp. 510-516, describes using an on-line inspection system to provide immediate feedback to an operator on solder paste quality for implementing a control loop. Bryant describes using a CCD camera to view solder pads and determine "image metrics," including such things as pad width, pad length, pad edge deviation, pad edge straightness, pad corner roundness, pad area, pad grayscale histogram, and pad grayscale image. The image metrics are used as the inputs to a neural network that grades the quality of a deposited solder pad on a scale from 0 to 9.
More recently it has been proposed to deposit solder pads by ejecting solder droplets from an ejector and directing the droplets to desired positions on a workpiece under the ejector.